Measurement of overvoltage

For the irreversible electrode (Fig. 34.4b), the null point is registered anywhere in the wide range of potential between (j) and 02. The slope of the curve is very small that is, io very small. An electrode with a large z o is therefore more reversible than one with a small z o. 

Before considering the theoretical ideas that relate the current to the overvoltage, we should understand the principle of the measurement of overvoltage. A cell is shown schematically in Fig. 34.5. A measured current is passed between the two electrodes A and B. The reference electrode R is the same kind of electrode as B. Matters are arranged so that the same electrode equilibrium is established at both B and R. When i = 0,B and R both have the same potential. When the current passes into B, this electrode has a potential measured on the potentiometer P that is different from that of R, which carries no current. This difference in potential is the measured overvoltage, rjm = The value of rj  

As the experiment stands, the measured value contains an ohmic component from the ir drop between R and B, a concentration component resulting from concentration changes in the vicinity of the electrode, and a component, denoted by rj, which is related to the rate constant of the reaction. Thus 

There are methods for measuring ohmic separately conc can usually be reduced to a negligible value by vigorous stirring. Thus from the value of rj can be found as a function of the current density. This rj, which is related to the rate constant of the reaction, is often called the activation overvoltage. 

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B) Apparatus for measurement of overvoltage at an electrode as a function of current density. 

For the measurement of overvoltage at definite c.d. s, the method adopted is, in principle, the same as that described on page 436, employing a cathode of known area. The solution should be completely free from dissolved oxygen or other reducible material for this reason it... 

Piontelli R (1955) Basis and examples of applications of new methods for measurement of overvoltages. Z Elektrochem 59 778-784... 

SO that the concentration of [Zn ] under the same conditions will be 10 g-molecule/L. With these ionic concentrations, the deposition potentials of copper and zinc in the absence of any polarization can each be calculated from Eq. (11.1) to be about —1.30 V. It should be mentioned here again that in practice, Eq. (11.1) refers to reversible equilibrium, a condition in which no net reaction takes place. In practice, electrode reactions are irreversible to an extent. This makes the potential of the anode more noble and the cathode potential less noble than their static potentials calculated from (11.1). The overvoltage is a measure of the degree of the irreversibility, and the electrode is said to be polarized or to exhibit overpotential hence, Eq. (11.2). 

As measurement of minimum overvoltage is marked by an inherent error due to indefinable and thus problematic determination of the beginning of a visible evolution of gases, it is more exact to quote the values whioh are valid for a certain current density (see Tab. 13a, b). 

The Tafel equation implies that the overvoltage is a measure of the thermodynamic irreversibility of the electrode reaction, and it is associated with the slow step of the process. We distinguish some types of overvoltage depending on the type of slow reaction. 

The first factor determines the tendency for dissolution to occur while the second and third, which are closely related, determine the rate of dissolution. The use of the standard electrode potentials as a measure of nobility is well known. The recognition that the exchange current density is a measure of the reversibility of a process and therefore a quantity characteristic of the reactivity of the system is more recent (13,32). As indicated by the Tafel relations, the exchange current density is a direct measure of the rate of the electrode reaction for any given value of the activation overvoltage (33). The values of iG may then be taken as a criterion for the electrochemical activity of a system. 

In this equation, and represent the surface concentrations of the oxidized and reduced forms of the electroactive species, respectively k° is the standard rate constant for the heterogeneous electron transfer process at the standard potential (cm/sec) and oc is the symmetry factor, a parameter characterizing the symmetry of the energy barrier that has to be surpassed during charge transfer. In Equation (1.2), E represents the applied potential and E° is the formal electrode potential, usually close to the standard electrode potential. The difference E-E° represents the overvoltage, a measure of the extra energy imparted to the electrode beyond the equilibrium potential for the reaction. Note that the Butler-Volmer equation reduces to the Nernst equation when the current is equal to zero (i.e., under equilibrium conditions) and when the reaction is very fast (i.e., when k° tends to approach oo). The latter is the condition of reversibility (Oldham and Myland, 1994 Rolison, 1995). 

They obtained a logarithm of current and potential relationship composed of two linear parts, and rationalized the polarization data. They determined the transfer coefficient in tire higher overpotential region 0.25 confinned later by Hori and Suzuki in their measurements of the partial current densities of HCOO formation at a Hg pool electrode.Tliis value indicates that tire rate determining step is tire first electron transfer to form CO2-" anion radical. The transfer coefficient in the lower overvoltage region is 0.67. 

In the galvanostatic method, constant current steps are applied for a predetermined duration and to measure the overvoltage. The polarization resistance curve is constructed by plotting the values of the overvoltage as a function of applied current for each current step. The slope at the origin of the plot gives the polarization resistance. Because the metal... 

Usually, the eleetrode reaetions (4.1) and (4.2) are moved away from equilibrium. For eaeh of the reaetions the potential is moved away from the equilibrium potential as a result of the net electrode reaetion oeeurring, i.e. a net electric current flowing through the interface between metal and liquid. The deviation from equilibrium is called polarization, and we say that the electrode is polarized. A measure of polarization is the overvoltage, i.e. the difference between the real potential and die equilibrium potential. When a corrosion process takes place on a surface, the real potential adopts a value somewhere between the equilibrium potential of the cathodic and anodic reactions, respectively, as illustrated in Figure 4.1. 

Therefore, for what concerns the primary distribution, we may conclude that the precision obtained with the BEM is better than we could ever measure and that precision will be as good or even better for secondary distributions because of the smoothing effect of overvoltages (see also section 3.. 2.4,). 

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